The pathogenesis and treatment of retinal vascular lesions of diabetes are to be investigated in dogs, a species known to develop microvascular lesions indistinguishable from those seen in diabetes mellitus in patients. The evolution of retinopathy is to be studied ophthalmoscopically and histologically in diabetic dogs, and in a new model of diabetic reginopathy discovered in experimentally galactosemic dogs. Diabetic animals and galactosemic animals are to be compared with respect to changes occurring in tissue morphology, biochemistry and physiology to identify abnormalities associated with and possibly responsible for the retinopathy. The potential role of excessive polyol production in the pathogenesis of diabetic retinopathy is being examined, and effects of administration of an aldose reductase inhibitor on the development of retinal and related complications are being investigated in the diabetic dog and galactosemic dog models. Metabolic activity of isolated retinal microvessels is to be documented and compared with that of cerebral microvessels. The extent to which early retinopathy can be arrested or reversed by correction of hyperglycemia is to be evaluated in galactosemic dogs for comparison with similar studies by us currently in diabetic dogs.